Multiple semiconductor devices are fabricated in a matrix on a semiconductor wafer, which is typically made of material such as sapphire, copper, silicon, and/or their compounds. The semiconductor wafer is then cut by a laser to divide or assist in dividing the semiconductor devices into separate pieces. Laser singulation may include any of the following processes: i) laser scribing, in which linear grooves (or scribe lines) are formed on the semiconductor wafer surface to facilitate breakage along the grooves; or ii) laser cutting, in which the semiconductor wafer is cut through from its top surface to its bottom surface.
Laser singulation is contingent on delivering irradiance (i.e. fluence or energy) to the semiconductor wafer that exceeds its material ablation threshold. By focusing a Gaussian laser beam using an objective lens, a laser output width of the Gaussian laser beam can be made small in the order of 1 to 20 μm. Such dimensions of the laser beam ensure that its irradiance exceeds the material ablation threshold of the semiconductor wafer for laser singulation.
However, when the laser beam width is made small, it is important to ensure that a distance between two consecutive laser pulses is within a maximum possible distance Dpulse in order to effect singulation. The relation between the maximum possible distance Dpulse of two consecutive laser pulses, the feeding speed Vfeeding of the laser beam, and the pulse repetition frequency fpulse of the laser beam is governed by the following equation:Dpulse=Vfeeding/fpulse (measured in units of mm/pulse or μm/pulse)
It is therefore seen that the feeding speed Vfeeding of the laser beam is constrained by the maximum possible distance Dpulse. One way to increase the feeding speed Vfeeding of the laser beam is by increasing its pulse repetition frequency fpulse. However, although the laser beam gives higher average power at higher pulse reception frequencies fpulse, its pulse energy drops rapidly as its pulse repetition frequency fpulse exceeds a certain threshold. Accordingly the feeding speed Vfeeding of the laser beam is ultimately limited by the constraints of both its optimum pulse repetition frequency fpulse and the maximum possible distance Dpulse.
In addition, scribe lines on the semiconductor wafer as formed by the Gaussian laser beam typically have a trough-like scribe depth along the scribing direction. This is because the irradiance distribution of the laser beam is of a Gaussian nature. Accordingly, portions of the scribe line that receives a weaker irradiance from the laser beam will have smaller depths compared with other portions that receive a stronger irradiance. To ensure a consistent scribe depth along the entire linear groove, further constraints may have to be imposed on the feeding speed of the laser beam.
Thus, it is an object of this invention to relax the aforesaid constraints on the feeding speed of the laser output to improve overall throughput.